Abstract
BACKGROUND: Phosphate is a fundamental nutrient for all creatures. It is thus not surprising that a single bacterium carries different transport systems for this molecule, each usually operating under different environmental conditions. The phosphonate transport system of E. coli K-12 is cryptic due to an 8 bp insertion in the phnE ORF. RESULTS: Here we report that an E. coli K-12 strain carrying the triple knockout ΔpitA Δpst Δugp reverted the phnE mutation when plated on complex medium containing phosphate as the main phosphorus source. It is also shown that PhnCDE takes up orthophosphate with transport kinetics compatible with that of the canonical transport system PitA and that Pi-uptake via PhnCDE is sufficient to enable bacterial growth. Ugp, a glycerol phosphate transporter, is unable to take up phosphate. CONCLUSIONS: The phosphonate transport system, which is normally cryptic in E. coli laboratory strains is activated upon selection in rich medium and takes up orthophosphate in the absence of the two canonical phosphate-uptake systems. Based on these findings, the PhnCDE system can be considered a genuine phosphate transport system.